Galactofuranose (Galf) residues are present in many pathogens. For example, they are essential components of the arabinogalactan layer of mycobacteria. Mycobacteria cause a number of diseases, the most deadly of these is tuberculosis (TB). Each year, Mycobacterium tuberculosis is responsible for 8 million human infections and 2 million deaths. (Tripathi, R. P.; Tewari, N.; Dwivedi, N.; Tiwari, V. K. Med. Res. Rev. 2005, 25, 93-131.) Strains have emerged that are resistant to most or all known antibiotics. (Marris, E. Nature 2006, 443, 131.) Resistance can be combated by developing an inhibitor with a new mechanism of action against a known target. (Sullivan, T. J.; Truglio, J. J.; Boyne, M. E.; Novichenok, P.; Zhang, X.; Stratton, C. F.; L1, H. J.; Kaur, T.; Amin, A.; Johnson, F.; Slayden, R. A.; Kisker, C.; Tonge, P. J. ACS Chem. Biol. 2006, 1, 43-53.) An alternative approach is to identify novel targets. One such potential target is the essential enzyme responsible for the incorporation of galacto-furanose residues, uridine 5′-diphosphate (UDP) galactopyranose mutase (UGM). (Lowary, T. L. Curr. Opin. Chem. Biol. 2003, 7, 749-756.)
UGM uses a unique mechanism to catalyze the isomerization of UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf) (Scheme 1, where U is uracil). (Soltero-Higgin, M.; Carlson, E. E.; Gruber, T. D.; Kiessling, L. L. Nat. Struct. Mol. Biol. 2004, 11, 539-543; Chad, J. M.; Sarathy, K. P.; Gruber, T. D.; Addala, E.; Kiessling, L. L.; Sanders, D. A. R. Biochemistry 2007, 46, 6723-6732.) Since UGM is not a target of other tuberculosis drugs, compounds that block UGM are expected to be effective against drug resistant strains.

The gene encoding UGM is essential for mycobacterial viability; the identification of UGM inhibitors can validate it as a therapeutic target. (Pan, F.; Jackson, M.; Ma, Y. F.; McNeil, M. J. Bacteriol. 2001, 183, 3991-3998.) Moreover, Galf residues are also found in some eukaryotes, including a number of pathogenic (to humans and animals) eukaryotes, therefore, UGM inhibitors can also provide insight into the role of Galf-containing oligosaccharides in these organisms. (Beverley, S. M.; Owens, K. L.; Showalter, M.; Griffith, C. L.; Doering, T. L.; Jones, V. C.; McNeil, M. R. Eukaryotic Cell 2005, 4, 1147-1154.) Additionally, there is no enzyme comparable to UGM in humans or other mammals increasing the appeal of UGM inhibitors as useful therapeutics.
Most efforts to develop UGM inhibitors have focused on UDP-sugar substrate analogs. (Caravano, A.; Dohi, H.; Sinay, P.; Vincent, S. P. Chem.-Eur. J. 2006, 12, 3114-3123; Liautard, V.; Christina, A. E.; Desvergnes, V.; Martin, O. R. J. Org. Chem. 2006, 71, 7337-7345; Ghavami, A.; Chen, J. J. W.; Pinto, B. M. Carbohydr. Res. 2004, 339, 401-407; Lee, R. E.; Smith, M. D.; Pickering, L.; Fleet, G. W. J. Tetrahedron Lett. 1999, 40, 8689-8692; Liautard, V.; Desvergnes, V.; Martin, O. R. Org. Lett. 2006, 8, 1299-1302.) Simple sugar derivatives, including galactopyranose or galactofuranose analogs, bind weakly with affinities in the millimolar range (Lee, R. E.; Smith, M. D.; Nash, R. J.; Griffiths, R. C.; McNeil, M.; Grewal, R. K.; Yan, W. X.; Besra, G. S.; Brennan, P. J.; Fleet, G. W. J. Tetrahedron Lett. 1997, 38, 6733-6736; Veerapen, N.; Yuan, Y.; Sanders, D. A. R.; Pinto, B. M. Carbohydr. Res. 2004, 339, 2205-2217.) Inhibitors that incorporate the uridine portion of the substrate bind substantially better, with affinities that approximate that of UDP-Galp (Kd=52 μM) (Itoh, K.; Huang, Z. S.; Liu, H. W. Org. Lett. 2007, 9, 879-882; Caravano, A.; Vincent, S. P.; Sinay, P. Chem. Commun. 2004, 1216-1217; Caravano, A.; Mengin-Lecreulx, D.; Brondello, J. M.; Vincent, S. P.; Sinay, P. Chem.-Eur. J. 2003, 9, 5888-5898; Pan, W. D.; Ansiaux, C.; Vincent, S. P. Tetrahedron Lett. 2007, 48, 4353-4356; Scherman, M. S.; Winans, K. A.; Stern, R. J.; Jones, V.; Bertozzi, C. R.; McNeil, M. R. Antimicrob. Agents Chemother. 2003, 47, 378-382.) These approaches have not yet afforded compounds that block mycobacterial growth.
Certain non-substrate based molecules have been identified as UMG ligands. For example, certain nitrofuranylamides have been identified as inhibitors of UGM catalysis and mycobacterial growth. (Tangallapally, R. P.; Yendapally, R.; Lee, R. E.; Hevener, K.; Jones, V. C.; Lenaerts, A. J. M.; McNeil, M. R.; Wang, Y. H.; Franzblau, S.; Lee, R. E. J. Med. Chem. 2004, 47, 5276-5283.) Nevertheless, the UGM inhibition and antimycobacterial activity of these compounds were not correlated, so they do not address the utility of inhibiting UGM.
Published International application WO 2005/007625 (Lee et al.), as well as published U.S. application 20050222408, relate to certain heterocyclic amides with anti-tuberculosis activity. More specifically, these patent documents relate to compounds of formula:
wherein A is selected from the group consisting of oxygen, sulfur, and NR15, and R15 is selected from the group consisting of H, alkyl, aryl, substituted alkyl, and substituted aryl; B, D, and E are each independently selected from the group consisting of CH, nitrogen, sulfur and oxygen; R1 is selected from the group consisting of nitro, halo, alkyl ester, arylsulfanyl, arylsulfinyl, arylsulfonyl and sulfonic acid; t is an integer from 1 to 3; and X is a substituted amide. These patent documents are incorporated by reference herein at least in part for the definitions of structural elements of the above formula.
A high-throughput, fluorescence polarization (FP) screen was reported to identify several compounds (including 1A-4A below) with good IC50 values (˜10−6 M) for the UGM from Klebsiella pneumoniae (UGMkleb) or M. tuberculosis (UGMmyco). (Soltero-Higgin, M.; Carlson, E. E.; Phillips, J. H.; Kiessling, L. L. J. Am. Chem. Soc. 2004, 126, 10532-10533.)

A directed library containing a 5-arylidine-2-thioxo-4-thiazolidinone core has been reported to identify factors influencing UGM ligand binding. (Carlson, E. E.; May, J. F.; Kiessling, L. L. Chem. Biol. 2006, 13, 825-837.) Several thiazolidinone derivatives were reported to be ligands for both the UGMkleb and UGMmyco homologs. The thiazolidinone scaffold, however, reacts reversibly with biologically relevant thiols in solution. Not surprisingly, inhibitors of this structural class were reported to fail to block mycobacterial growth.
There remains a need in the art for small molecules that exhibit antimicrobial activity, particularly against Mycobacteria. 